arm64: Fix kernel panic in get_arm64_sve during core dump

[wac_gmail.com]

Summary

The coredump logic calls get_arm64_sve twice: once to get the note size, and once to get the data. The note size calculation depended on the volatile PCB_FP_SVEVALID flag. If this flag was cleared between the two calls (e.g., due to a context switch clearing the flag to comply with the ABI), the second call would expect a smaller buffer size than the first, triggering a KASSERT panic ("invalid size").

Fix this by ensuring the SVE state is saved to the PCB before we decide whether to use SVE or VFP.

PR: 292195

Test Plan

Triggered core dump repro from bug 292177 and observed a good core file and no kernel panic.

[andrew]

Would it be enough to move the td == curthread .. check & call to vfp_save_state before checking for PCB_FP_SVEVALID in line 942?

[wac_gmail.com]

I don't think moving vfp_save_state(td) is sufficient because of lazy SVE restoration. If the thread hasn't executed SVE instructions since a context switch, the trap to re-enable the unit won't have fired. In that state, vfp_save_state(td) sees the unit as disabled and won't set PCB_FP_SVEVALID and we'll still potentially see this panic. Checking pcb_svesaved != NULL avoids this by checking the backing store directly.

[andrew]

After the first call to vfp_save_state from get_arm64_sve with buf == NULL the PCB_FP_SVEVALID flag will be set correctly based on if pcb_svesaved contains valid data or not (it may be clear if called while in a syscall). If we then context switch then vfp_save_state_switch will exit early as VFP is disabled so PCB_FP_SVEVALID isn't changed & later calls to vfp_save_state will also not change the state.

It's ok to drop the SVE state if the thread is in a system call (other than sigreturn) as the ABI doesn't guarantee it will remain valid.

[andrew]

I tested a patch to move vfp_save_state & haven't hit the KASSERT, even when loading the system down to try cause more context switching.

[wac_gmail.com]

Incorporated your suggested change just moving the vfp_save_state and tested successfully. Thank you for your patience on this.

[andrew]

Can you send ma a git patch with this change? e.g. the output of git format-patch